This invention relates in general to smoke detection and in particular to the operation and supplying of power to smoke detectors.
Recognition in recent years that deaths and injuries from fires, particularly in residential buildings, are far more frequently due to smoke rather than from actual flames has accelerated the development of smoke detectors of various kinds. Work has also been spurred on by the passage of laws and regulations requiring the use of smoke detectors in residences being newly constructed or renovated.
The most common types of smoke detectors presently in use perform the actual detection of smoke by either optical or ionization devices. The present invention is concerned with both types of detectors in several of its aspects but, as is noted below, may best be understood in connection with optical detectors and will be so described.
The basic principle underlying optical smoke detection is the reflection or scattering of light by smoke particles. Simply stated, the absence or presence of a certain amount of light from a source is continuously monitored by a photosensitive element. Smoke, if it appears, is permitted to pass between the light source and the photosensitive element and its effect on light transmission changes the response of the photosensitive element in one way or another to trigger an alarm. Commonly, it is the reflection of light by smoke particles to the photosensitive element which initiates the change of response.
In recent years, the most notable improvement in optical smoke detectors has been the substitution of light-emitting diodes for incandescent light bulbs as the light source in the detectors. Although the light-emitting diode (LED) is a reliable and durable light source, it has certain disadvantages, not the least of which is that it is an inefficient source of light. Consumption of power, which is especially important in battery-operated systems, is unfortunately high. Also, because the light output is relatively low, ultra-sensitive receiver devices and circuits are needed.
One other area of investigation to improve efficiency of optical smoke detectors has involved the use of pulsating light sources. In such arrangements, the use of a duty cycle which cuts the time of operation of the light without sacrificing smoke detecting capacity has indeed resulted in reduced power consumption, but much room for improvement remains.